X-linked mental retardation with Xq27 chromosomal fragile site (fragile (x)) is an important and perplexing genetic disorder. The disease is 1) common, affecting one in 2000 males, 2) inherited in a peculiar pattern as an apparently X-linked trait with significant numbers of non-penetrant hemizygous males, and 3) associated with a cytogenetic abnormality, a fragile site at Xq27, which must be induced by interfering with DNA replication through pyrimidine nucleotide triphosphate deprivation. 1. We will use Chinese hamster cells already deficient in hypoxanthine phosphoribosyltransferase (HPRT) and thymidylate synthase to select for mutations in the DNA repair enzyme uracil-N-glycosylase. 2. We will isolate HPRT-cell lines from Chinese hamster cells already defective in de novo purine and pyrimidine biosynthesis. 3. We will make somatic cell hybrids between lymphoblasts from fragile (X) patients and rodent cells with defective purine biosynthesis, pyrimidine biosynthesis and uracil-N-glycosylase activity, using the HPRT-phenotype of the rodent cells to select and maintain the fragile (X) in hybrids. 4. We will carry out fragile (X) expression studies in these hybrids by depriving them of the appropriate purine or pyrimidine precursors. In this way, we can test whether slowing of DNA replication by limiting purine or pyrimidine precursors leads to fragile site expression, or whether a more specific pyrimidine deprivation or repair of misincorporated uracil are responsible for the cytogenetic phenomenon at the Xq27 fragile site.